1. Field of the Invention
The present invention relates to a 3-dimensional P-N junction solar cell and the preparation method of the same.
2. Description of the Related Art
The solar cell that can generate electricity directly from the sunlight is the most promising future energy production method, considering it is advantageous for producing a clear energy safely. To construct such a solar cell, a variety of inorganic and organic semiconductors have been applied. However, the most representative products succeeded in the commercialization so far are the silicon solar cell comprising silicon (Si) as a major material and the CIGS thin film solar cell.
The said silicon solar cell has the advantage of high photo conversion efficiency but has the disadvantage of high production costs. As an alternative, a thin film solar cell is attracting our interests which uses a compound semiconductor where a thinner layer can be applied. The representative thin film solar cell is such a thin film solar cell that comprises IB elements (CIS or CIGS), IIIA elements, and VIA elements as the light absorption layer.
The major components of the thin film solar cell are the light absorption thin film generally composed of Cu(In,Ga)Se2 and the buffer thin film composed of Cds or other n-type compound semiconductors. In particular, the CIS or CIGS light absorption layer is the most critical factor that affects the performance of such a solar cell.
The CIS or CIGS light absorption layer is generally produced by co-evaporation method or vacuum evaporation method such as sputtering using an expensive vacuum equipment. And recently solution process such as printing has been studied in order to lower the manufacturing costs of the CIS or CIGS thin layer solar cell and also to make the area large.
However, since the highly expensive raw materials such as In and Ga are necessary for the production of CIS or CIGS thin film solar cell, the effort to lower the manufacturing costs with the raw materials is limited.
Instead of In and Ga, the compound Cu2ZnSnS4 (CZTS) or Cu2ZnSnSe4 (CZTSe) containing Zn and Sn which are distributed rich on the earth and less hazardous has a highly preferable optical properties appropriate for the solar cell (for example, optical absorption coefficient: >10−4 cm, band gap: 1.5 eV), so that it came in to the spotlight as a next generation thin film solar cell material that could replace the CIGS thin film solar cell.
There are two kinds of methods to prepare the CZTS thin film, which are vacuum process and non-vacuum process.
The vacuum process includes sputtering and co-evaporation method. This method has the advantage of easiness in controlling the chemical composition, phase behavior of the thin film, microstructure of the thin film, and reproducibility, but has the disadvantage of high costs.
In the meantime, the non-vacuum process can provide a solar cell with an inexpensive price, which includes spin coating, spraying, and nanoparticle based method.
To perform the non-vacuum process, an organic binder is necessary to secure the coating property, and the resulting carbon residue is a problem. The morphology of the thin film is also limited in this method.
In general, P-N junction solar cell is produced by forming the P-N junction between P type semiconductor and N type semiconductor. Once exposed on a strong light, electrons and positive holes are excited and move freely in the semiconductor, and then the electrons and positive holes move to each electrode by P-N junction to generate electric current.
To produce a high performance solar cell, it is necessary to make the movement of the electrons and positive holes smooth in the semiconductor. To do so, a strategy either to increase the diffusion length of a carrier so as to make the movement of the electrons and positive holes smooth or to deliver a carrier to the electrode more efficiently is necessary. Therefore, the studies concerning a high quality solar cell light absorption layer forming technique have been actively going on.
Korean Patent No 10-1149474 presents the preparation method of a CIS or CZTS compound thin film which is used as the light absorption layer of a solar cell. Precisely, the method for preparing a CIS or CZTS compound thin film via solution process is described therein.
However, it is still required to improve the efficiency of such a CZTS solar cell, compared with other types of solar cells.
In the course of study to establish a solar cell with a simpler preparation process but with a high efficiency, the present inventors succeeded in the preparation of a nano level 3-dimensional P-N junction solar cell wherein a 3-dimensional porous P type semiconductor thin film is formed and the surface of the crystal grains of the P type semiconductor thin film is coated by the N type buffer layer, and further confirmed that this solar cell could be able to not just overcome the limited efficiency but increase the efficiency of the conventional thin film solar cell by making the carrier formed by a light delivered smoothly, leading to the completion of the P-N junction solar cell of the invention.